Homework 4 Chapter 4: How Do Switches and Routers Find Paths

droppercauseNetworking and Communications

Oct 28, 2013 (3 years and 9 months ago)


Homework 4

Chapter 4: How Do Switches and Routers Find Paths


Routers and bridges use the concept of metric or cost.


Discuss how routers and bridges use path determination. What are the

Routers evaluate the metric of an entire end
end route or path. Bridges take a
more local forwarding view than do routers.


Routing Protocols provide information for forwarding the packets of a routed


List the Dynamic routing protocols.

Routing Information Protocol (RIP)

Interior Gateway

Routing Protocol (IGRP)

Enhanced Interior Gateway Routing Protocol (EIGRP)

Intermediate System to Intermediate System (IS

Border Gateway Protocol (BGP)


What does a router have to do before it forwards?

Obtain information about potential routes

cipate in exchanges for dynamic routing protocols

Install the best of these routes in the RIB.

Look up the destination address of incoming packets in the FIB.


Can a RIB apply filtering?

Yes, it can

as an option


Define the Classic Route Lookup Criteria

The Classic Route Lookup Criteria consists of four parts:

1. Basic Match: The network address prefix is extracted from the packet.
Any non
matching route is rejected.

2. Longest Match: If there are multiple routes that match the basic match,
the rou
te with the highest number of matching bits is selected.

3. Weak Type of Service: Although outdated, the TOS bit fields in the IP
header are considered for packet prioritization. This does not affect the route

4. Best metric: Routes are assign
ed metrics based on either administrative
distance for distance vector routing protocols or factors such as hop count,
bandwidth utilization, MTU, etc for link state protocols. The route with the best
metric is chosen. The best metric is usually the lowe
st metric.


What is a default route?

A default route is the route selected when there are no other matching routes in
the routing table. If there is no default route set, a “Destination unreachable”
ICMP message is returned.


What is the address of the d
efault route?

Address: Mask: Next Hop:
Next Hop address here

In CIDR notation:


Layer 3 addresses have two levels of hierarchy. What are they?

Prefix and Host


Does a routing table cause problems because of the size? Why an
d what can you

The size of a routing table can cause problems due to increased router CPU
utilization as the table grows. Without a hierarchy, it is necessary to search every
address table in the domain. Large routing tables also cause problems with

convergence in routing protocols.

Default routes and route summarization are two methods of controlling the size of
routing tables. Default routes assume that an upstream router knows more about
the desired path than it does and so it forwards pack
ets bound for unknown
addresses to the default route.


Draw and discuss a default routing scenario.

: Default routing scenario


How do routes get installed in the RIB?

A route must pass thr
ough acceptance filters. The RIB is checked for duplicates
and assuming none are found the route is inserted into the RIB.


What does administrative distance mean? What number takes on the higher

Administrative distance is the preference given

to a route. Administrative
distances are numbered from 0

255. 0 means that a route is directly connected
or is referenced by interface. Static routes usually an administrative distance of 1.
Different routing protocols have different metrics though
they can be changed if
necessary although doing so without very good reason is not recommended.


What must routes have to be considered for load sharing?

Routes must have the same administrative distance and must have the same metric
if placed in the RIB
by a dynamic routing protocol.


List the factors used in computing metrics.

Link bandwidth, link delay, Administrative preferences, link error rate, and link


What is a quasi
static route?

static routes are static routes with differen
t administrative preferences that
can be used for load


Do quasi
static routes need to be configured?

static routes must be manually configured.


What can you do to administer your edge routers?

1. Try very hard to avoid duplicate static r

2. Automate the updating of your router configurations using configurations
created from your user address assignment database and loaded via tftp.

3. Generate human readable reports for troubleshooting and address allocation


Layer 2 Paths

Layer 2 Paths specify where not to go to find a particular destination. Frames are
forwarded over interfaces that are not blocked by the spanning tree algorithm.


What are Layer 2 Paths used for?

Layer 2 paths use the spanning tree algori
thm to block forwarding in such a manner
as to prevent looping.


Discuss the Spanning Tree Algorithm and how priority in the bridges are

The Spanning Tree algorithm is initiated by setting all ports to blocking. As ports
come on
line, they ar
e transitioned from a blocking state to a Learning /
Forwarding state and finally to a listening state. Priorities in bridges are
determined by concatenating the priority number and bridge ID. This is
compared among adjacent bridges and the lowest priori
ty/bridge ID combination
is given priority. The bridge with the highest overall priority is the root bridge.


How are routes discovered using VLANs and ELANs?

There are no special path determination processes. VLANs and ELANs
encapsulate a payload proto
col and associate it with a VLAN identifier.


List the tunneling components.

1. Original payload information

2. Tunneling information

3. Delivery protocol


What is a trunk?

A trunk is a path between switches or bridges that can differentiate between
tiple VLANs on the same link.


What are the two kinds of VLAN trunking?

1. Tunneling method

2. VLAN tagging